Remove Requirements API

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This issue must follow the deprecation of the Requirements API for at least 6 months. See #345308.

When requirements are migrated to issues (#323779 (closed)) and internally we use this issue when listing/managing requirements through Requirements API (#323781 (closed)) then we can start deprecating Requirement graphql API.

Before it's removed we have to make sure that:

• deprecation should follow graphql API deprecation process https://docs.gitlab.com/ee/api/graphql/#deprecation-and-removal-process
• issue's API supports all features we need for interacting with requirement issue type
• UI is updated to use the existing issues graphql API endpoints (#343408 (closed))